1. Field of the Invention
The present invention relates to film cartridges employing roll film provided with a protective backing strip, and, more particularly, the invention relates to improving the reliability of such film cartridges with respect to maintaining the flatness of the film in its exposure plane.
2. Description of the Prior Art
U.S. Pat. No. 3,138,084, issued in my name on June 23, 1964, is directed to the general type of film cartridge to which the present invention relates and describes in considerable detail the film flatness problem inherent in such a cartridge because of the use of an opaque backing strip. Briefly, this problem is attributable to the difference in radii of the film and backing strip at their respective points of tangency to the supply and take-up rolls and to the varying ratio between the radii of the supply and take-up rolls during progressive stages of film advancement.
Because the backing strip is wound outside the film, though both are wound together, each convolution of backing strip is slightly longer than the corresponding convolution of film. Although not intuitively apparent, the fact that the circumference of a convolution is equal to .pi. times its effective diameter dictates that the difference in length between the corresponding film and backing strip convolutions is a function only of the effective thickness of those convolutions and is independent of the diameter of the roll. In other words, assuming that the roll is tightly wound, the backing strip surplus generated by each rotation of the supply roll is equal to the extra length of backing strip required for each rotation of the take-up roll, regardless of the diameters of those rolls. However, unless the two rolls are of exactly the same diameter, one roll rotates more than the other whenever film is advanced. Consequently, if the supply roll is larger than the take-up roll, a backing strip shortage occurs between the rolls and, if the supply roll is smaller than the take-up roll, a paper excess will occur. In a compact film cartridge of the type under consideration, surplus film, created by a backing strip shortage, cannot be absorbed readily and will result in buckling of the film in the exposure plane.
In accordance with previously cited U.S. Pat. No. 3,138,084, this situation is addressed in conventional 126 type film cartridges by choosing the dimensions of the cartridge and film components such that enough surplus backing strip is generated by the initial unwinding of the supply roll, before the film reaches the takeup roll, to make up for the surplus film generated thereafter before the take-up roll becomes as large as the supply roll. This approach has proven to be quite satisfactory under most circumstances. However, in some cases, the supply roll may "clock spring" in its chamber after the film has been partially advanced. If this occurs, the backing strip and film in the supply chamber slip longitudinally relative to one another as the supply roll enlarges to fill the supply chamber. Consequently, the supply roll is again larger than the take-up roll and film flatness in the exposure area is thereafter impaired by a backing strip shortage. This occurance can arise, for example, if the film and backing strip initially do not have proper stiffness or "curl" characteristics and/or if the cartridge is exposed to excessive vibrations or temperature conditions.
One approach to the potential "clock springing" problem is disclosed in British Pat. No. 1,112,349, published on May 1, 1968, which proposes confining the supply roll by means of a resilient spring or pad so that it cannot "clock spring" in the film supply chamber. This approach, however, requires the use of at least one additional cartridge component and poses the possibility of producing pressure marks on the film and/or undesirably increasing the force required to advance the film.
Another approach to the potential backing strip shortage problem is proposed by U.S. Pat. No. 3,614,012, issued on Oct. 19, 1971, and is used in conventional 110 type film cartridges. In accordance with this approach, the film take-up core is substantially as large in diameter as the interior of the film supply chamber so that the take-up roll is always larger than the supply roll. However, this approach is incompatible with improving the film flatness characteristics of conventional 126 type film cartridges, because it would require increasing the size of the film take-up chamber and would thus make the improved cartridge unusable in existing cameras.
The foregoing considerations are directed to eliminating a "net" shortage of backing strip, which means that there is less backing strip than film extending between the two rolls. Even if such a net shortage is theoretically eliminated by one of these approaches, a surplus of film can nevertheless occur in the film exposure region. This condition can arise because the backing strip surplus is generated in the supply chamber rather than in the take-up chamber. Accordingly, the backing strip may be relatively taut between the supply chamber and the take-up roll, and, therefore, is not completely compliant to the film. Also, because the backing strip moves between the rolls slightly faster than does the film, its frictional engagement with the film tends to move surplus film from the supply chamber into the exposure region. This situation, in conventional roll film cameras, is recognized in U.S. Pat. No. 1,170,674 issued on Feb. 8, 1916, which proposed to eliminate the problem by providing the backing strip, intermediate the film ends, with a longitudinally expansible section comprising a tuck or double fold in the backing strip. In such cameras, the film was simply stretched over a support frame to provide a relatively flat exposure region and there was adequate room behind the film in the exposure region to accommodate the passage and unfolding of the expansible backing strip section. This approach is not applicable to a film cartridge of the type to which the present invention is directed, because the tuck or folded section of the backing strip would almost certainly jam in a film guide channel or throat of such a cartridge.